I. Field of the Invention
The present invention relates to the heat treating of ferrous articles. In particular, the present invention relates to a heat treating control process wherein ferrous articles are treated in a mixture of a gaseous carbon source and an inert carrier gas.
II. Description of Prior Art
Copies of the following prior art references were appended to the original application papers and discussed in a Prior Art Statement.
L'Air U.S. Pat. No. 4,035,203. This patent discloses a process which introduces nitrogen and methane into a heat treating furnace and has an analyzer for the methane level within the furnace. The methane level within the furnace is automatically regulated in response to the analyzer. The L'Air process does not measure, analyze or control the level of decarburizing agents in the furnace. Also, L'Air process does not control the carbon monoxide level of the furnace.
Air Products U.S. Pat. No. 4,049,472. This patent discloses a process wherein a gaseous mixture is prepared at ambient temperatures and introduced into the furnace. The gaseous mixture comprises: 62-98% nitrogen, 1.5-30% methane (natural gas), 0.2-15% carbon dioxide and 0-10% ammonia (if carbonitriding).
The carbon potential within the furnace is determined according to a ratio of methane to carbon dioxide. The patent process requires a certain level of carbon dioxide to control the carbon potential within the furnace. This is a disadvantage since carbon dioxide is a strong decarburizing agent. No attempt is made to control the level of other decarburizing agents (oxygen and water vapor) within the furnace. Carbon monoxide levels are not measured.
Airco U.S. Pat. No. 4,049,473. Nitrogen is introduced during the Airco process only to the furnace vestibule, although nitrogen may be introduced into the furnace proper prior to carburizing to act as a purge. A hydrocarbon source such as methane is introduced into the furnace proper without a carrier gas. The carbon potential (i.e., the level of carbon in all compounds such as carbon monoxide and methane) is measured by an electric resistance wire which controls the introduction of natural gas into the furnace. The total carbon present within the furnace is measured--including the carbon in decarburizing agents such as carbon dioxide. Hence, the Airco process fails to analyze or control the level of decarburizing agents within the furnace. In fact, an affidavit filed by the applicants during the prosecution of this patent reveals that decarburizing agents such as oxygen contained in air must be specifically introduced into the furnace proper as an "adjustment" to assure that substantially all of the methane is reacted to avoid sooting.
Metal Progress (February, 1948, pages 241-246). This article discusses a furnace atmosphere created by the introduction of nitrogen and methane wherein the carbon monoxide level would be less than or about 1% and the carbon dioxide level would be essentially zero. See page 244. However, the article concludes that a measurable level of carbon dioxide is necessary to control the process. See pages 244 and 246. The article does not suggest that the nitrogen flow rate could be used to control the level of strong decarburizing agents which may, for example, result from air leaks into the furnace.
Metal Progress (October, 1977, pages 9-11 and June, 1978, page 96). The article discloses a heat treating process utilizing nitrogen and methane, the methane level being controlled by a methane analyzer. The accompanying letter to the editor raises a problem of controlling the decarburizing agents that may exist in the furnace atmosphere which leads to a large variation in the levels of carbon monoxide in the furnace atmosphere. However, no solution was offered for that problem, nor is the nitrogen flow controlled to maintain the control of carbon monoxide.